Method and device for hardening the inner surface of holes, in mechanical pieces of cast iron of predominantly ferritic matrix

ABSTRACT

A method and a device for hardening at least one portion of the inner surface ( 12 ) of a hole ( 11 ) made in a mechanical piece ( 10 ) of cast iron of predominantly ferritic matrix, comprising the step of subjecting said portion of the inner surface ( 12 ) to the action of a laser beam ( 100 ), until it causes the melting of its surface layer. In particular, said method comprises the steps of: projecting the laser beam ( 100 ) in axial direction inside the hole of the piece ( 10 ); deviating the laser beam ( 100 ) through a deviator means ( 4 ), so that the same is projected onto the inner surface of the hole itself, causing the melting of a surface layer; rotating the deviator means ( 4 ) or the piece ( 10 ) with respect to each other, and sliding in axial direction the deviator means ( 4 ) or the piece ( 10 ) with respect to each other, until the desired portion of the inner surface ( 12 ) is subjected to the laser beam ( 100 ).

The present invention generally refers to a method and device for thehardening of the inner surface of holes made in mechanical pieces iniron casting, and mechanically worked. It is known that formanufacturing these pieces, an easily workable cast iron is employed,such as the cast iron of predominantly ferritic matrix. More inparticular, the invention regards a method and device intended to beemployed in the process of manufacturing pump bodies of normal fluidpumps, to harden the surfaces of the cylindrical cavities adapted todefine the reception and guide seats of members in movement, such assliding pistons or rotors, so to avoid seizure.

As is known, the body pumps are realised by means of a forming process,which foresees pouring a metallic alloy inside appropriate dies in orderto obtain a rough casting. This is then subsequently subjected tomechanical working, which confers the necessary precision topredetermined surfaces and in particular to the above mentioned surfacesintended to be in contact with the members in movement.

For these reasons, the metallic alloy utilised in the manufacture of thepump bodies is, as said, a cast iron of predominantly ferritic matrix,easily workable in the machine tool, but little adapted to beingsubjected to a thermal hardening treatment.

The use of the pump at low ambient temperatures may lead to drawbacks.This occurs when the inner surfaces of the “cold” pump are struck withhot oil. The difference in mass between the pump body and the innermembers prevents a homogenous dilation of the system elements, favouringseizure phenomena, which often irreparably damages the pump itself.Presently, to overcome this drawback, an anti-seizure surface treatmentis used on the surfaces intended to be in contact, for example aphosphate treatment.

Nevertheless, the equipping of a phosphate plant is quite difficult, andmoreover leads to a considerable environmental impact.

Object of the present invention is, in general, that of superficiallyhardening the surfaces of holes made in mechanical pieces of cast ironof predominantly ferritic matrix and, more in particular, hardening thesurfaces of the cylindrical cavities of the pump bodies in contact withthe members in movement, so to effectively avoid the onset of seizurephenomena. Further object of the invention is to obtain a lasting,reliable hardening which may be realised in a simple, quick and economicmanner on a cast iron which is highly workable by the tool.

Such objects are achieved, according to the finding, with a lasertreatment of the relevant surfaces, until it causes the melting of theirsurface layer. The treatment may be conveniently localised, i.e. limitedto certain surface portions.

More precisely, the treatment foresees the steps of:

a) projecting a laser beam in axial direction inside the hole of thepiece in cast iron of predominantly ferritic matrix;

b) deviating the laser beam through a deviator means which is insertedin the hole, so that the same is projected on the inner surface of thehole itself, causing the surface melting;

c) rotating the deviator means or the piece with respect to each other;and

d) making the deviator means or the piece slide in axial direction withrespect to each other, so to subject at least one desired portion ofsaid inner surface of the hole;

In this manner, a thermal treatment of surface melting of the cast ironis carried out, obtaining a layer with homogenous and particularly hardstructure.

Indeed, the melting, which involves a very small thickness of thematerial, less than 0.5 mm, and the subsequent quick and spontaneouscooling, due to the great mass of material surrounding the melted zone,causes a change of phase in the metallic structure of the cast iron.

In particular, the carbon present in the form of graphite is largelydissolved in the melted metal matrix, which is enriched with carbon andsolidifies with predominantly ledeburitic structure, characterised byhigh hardness.

According to the invention, in particular, the treatment may beinterrupted at the surface portions having edges with specific roles,for example sealing functions in association with other components.

The treatment, in fact, may cause a slight local deformation of saidedges which therefore would not be fit for carrying out their function.

According to the invention, moreover, the aforesaid deviator comprises afaceted body which is realised with a material of high thermalconductivity, such as copper, whose appropriately prepared surfacepossesses a high reflecting power.

In particular, said surface is made specular by means of a surfacefinishing treatment, preferably by means of polishing; furthermore,during the treatment, it is properly cooled.

Further and advantageous characteristics of the invention will be clearfrom the reading of the following description, provided as merelyexemplifying and not limiting, with the air of the figures reproduced inthe attached drawing tables, in which:

FIG. 1 is a side schematic view of a device adapted to carry out thehardening treatment, in a manner in accordance with the invention;

FIG. 2 is a detail of the trace section II-II indicated in FIG. 1,enlarged and shown during the execution of the treatment;

FIG. 3 is the trace section III-III indicated in FIG. 2.

From the mentioned figures, a device 1 is noted which is adapted torealise, by means of a laser, a hardening treatment of the inner surface12 of a hole 11, realised in a mechanical piece 10 of cast iron ofpredominantly ferritic matrix (see also FIG. 3).

Such device 1 comprises a laser source 2, which is provided with anemission head 20 which projects a laser beam 100 (indicated with dashedline in FIGS. 2 and 3) along a predetermined direction.

The emission head 20 is firmly associated with a support 3, whichsupports a faceted copper cylinder 4 arranged along the emissiondirection of the laser beam 100 and adapted to deviate the latter bysubstantially 90°.

As is visible in FIG. 2, said faceted cylinder 4 has a first sectionaxially inserted within a containment sleeve 30 of the frame 3, and asecond section which projects from said containment sleeve 30 from thepart of the emission head 20.

In this manner, the axis of the faceted cylinder 4 lies substantially inthe emission direction of the laser beam 100 (see also FIG. 3), and thebottom face 40 of the cylinder 4 itself, which is made specular by meansof polishing, acts as a reflecting surface.

To avoid that, during the treatment, the faceted surface 4 is melted ordamaged when the reflecting surface 40 is struck by the laser beam 100,means (not shown) are foreseen which are adapted to cool it; said meansmay comprise, for example, channelling made inside the cylinder 4itself, in which a refrigerating liquid circulates.

Furthermore, as illustrated in FIGS. 1 and 2, the device 1 comprises asupply nozzle 6 which is adapted to direct, during the treatment, a jetof protective gas on the reflecting surface 40 of the cylinder 4 and onthe treated zone, in order to avoid oxidation; said protective gas beingcontained in an appropriate pressure cylinder (not shown), to which thenozzle 6 is connected.

As is shown in FIG. 2, the reflecting surface 40 is sloped 450, and ishit by the laser beam 100 so to reflect it with an angle of deviationequal to 900. In case it is necessary to reach particular positions,however, the slope of the aforesaid reflecting surface may also bedifferent from 450.

The mechanical piece 10 to be treated is mounted on a support andmovement device 5, which comprises a motorised carrier 50, sliding withreciprocating motion on a rectilinear guide 51.

In particular, said motorised carrier 50 is provided with a rotatingmandrel 52 adapted to support the cast iron piece 10 of predominantlyferritic matrix, so that the axis of the hole 11 lies on the rotationaxis of the mandrel 52 itself.

As is illustrated in FIGS. 1 and 3, the light source 2 and the movementdevice 5 are mutually positioned so that the rotation axis of themandrel 52, and therefore the hole axis 11, is parallel to the directionof the laser beam 100 leaving the emission head 20, and the mouth of thehole 11 is turned towards the laser source 2.

With reference to FIG. 2, at the beginning of the treatment, the carrier50 is advanced towards the laser source 2, and the faceted cylinder 4axially penetrates inside the hole 11, until it is positioned so to becapable of deviated the laser beam 100 on a first edge 12′ of thecylindrical surface 12 to harden (see also FIG. 3).

At this point, the source 2 is activated to generate the laser beam 100and, at the same time, the mandrel 52 is placed in rotation at apredetermined speed, and the carrier 50 is also driven to slide at apredetermined speed, until the laser beam 100 reaches the second edge12″ of the cylindrical surface to harden.

In this manner, the laser beam 100 eventually hits all the points ofsurface 12 of the hole 12, causing the melting of a layer of cast ironof predominantly ferritic matrix having a very small thickness, about0.5 mm.

Consequently, the metallic structure of said melted layer, coolingrapidly due to the proximity of a great quantity of non-melted material,undergoes-a change of state. The carbon in the form of graphite, presentin the cast iron of predominantly ferritic matrix, is dissolved in themetallic matrix, which is enriched with carbon and assumes a ledeburiticstructure to confer a very high hardness to the surface layer.

In order to obtain a quick and effective melting of the surface 12 ofthe hole 11, however, the treatment must be conducted respecting severaloptimal process parameters.

In particular, the thickness of the melted layer substantially dependson the maximum temperature reached by the surface 12 of the hole 11during the treatment, which in turn depends both on the energy of thelaser beam 100 which is absorbed in the time period by the cast iron ofpredominantly ferritic matrix of the piece 10 and, of course, by thetime of exposure of the same surface 12 to said laser beam 100.

Regarding the energy absorbed by the cast iron of predominantly ferriticmatrix, this is determined in particular by the physical properties ofthe laser beam 100, and by its geometrical characteristics.

Physical properties include: the wavelength of the laser beam 100, onwhich the absorption coefficient of the cast iron of predominantlyferritic matrix depends, that is the percentage of energy absorbed andutilised in an active manner for the treatment, with respect to thatreflected by the surface 12 to be treated; and the power of the laserray 100 itself.

Geometrical characteristics, on the other hand, include the focaldistance of the laser beam 100 leaving the source 2, and the dimensionsof the “spot”, that is the luminous point projected on the surface 12.

Regarding the time of exposure to the laser beam 100, finally, this is afunction of the speed with which the luminous “spot” moves with respectto the surface 12 to be treated, which depends of course on the rotationspeed and translation speed of the piece 10, with respect to the facetedcylinder 4.

It should therefore be underlined that, regarding energetic parameters,optimal results were obtained with a laser beam 100 of wavelengthcomprised between 800 nm and 950 nm, and power comprised between 1500 Wand 2200 W.

This along with a focal distance of the laser beam 100 comprised between66 mm and 500 mm, and a rectangular luminous “spot” with sides comprisedbetween 4 mm and 9 mm.

Regarding the exposure time, optimal results were demonstrated with aspeed of the luminous “spot”, with respect to the surface 12, comprisedbetween 20 mm/s and 40 mm/s.

Lastly, as merely exemplifying, the process parameters which permittedobtaining optimal results were reported, in terms of surface hardnessand treatment time, on the inner surface of a cylindrical cavity of apump body in cast iron of predominantly ferritic matrix GS400.

These parameters, which were compiled in the table placed at the end ofthe present description, were obtained during a series of experimentsconducted by the Applicant, in which there were used: a diode lasersource 2 adapted to emit a continuous laser beam 100; a faceted coppercylinder 4 with 20 mm diameter; a reflecting surface 40 sloped at 45°and superficially polished to have a roughness Ra<0.08; and a supplynozzle 6 of the protective gas which, connected to a pressure cylinder,directs the jet directly onto a reflecting surface 40, and onto thesurface 12 struck by the laser beam 100. Parameter Value LASER SOURCEROFIM DL022 LASER WAVELENGTH 808-940 nm ABSORBED POWER 6680 WATTS POWERSUPPLIED ˜2,000 WATTS FOCAL LENGTH 136 mm “SPOT” SIZE 4 × 9 mm “SPOT”SPEED 25 mm/s PROTECTIVE GAS NITROGREN

1. Method for hardening at least one portion of the inner surface (12)of a hole (11) made in a mechanical piece (10) of cast iron ofpredominantly ferritic matrix, characterised in that it comprises thestep of subjecting said portion of the inner surface (12) to the actionof a laser beam (100), until it causes the melting of its surface layer.2. Method according to claim 1, characterised in that it comprises thesteps of: a) projecting a laser beam (100) in axial direction inside thehole of the piece (10); b) deviating the laser beam through a deviatormeans (4) so that the same is projected onto the inner surface (12) ofthe hole itself, causing the melting of a surface layer; c) rotating thedeviator means (4) or the piece (10) with respect to each other; and d)making the deviator means (4) or the piece (10) slide in axial directionwith respect to each other, until said portion to be treated of theinner surface (12) is entirely subjected to the laser beam (100). 3.Method according to claim 1, characterised in that the thickness of themelted layer of cast iron of predominantly ferritic matrix is less than0.5 mm.
 4. Method according to claim 1, characterised in that thewavelength of the laser beam (100) is comprised between 800 nm and 950nm.
 5. Method according to claim 1, characterised in that the power ofthe laser beam (100) is comprised between 1500 W and 2200 W.
 6. Methodaccording to claim 1, characterised in that the focal distance of thelaser beam (100) is comprised between 66 mm and 500 mm.
 7. Methodaccording to claim 1, characterised in that the laser beam (100)generates on the inner surface (12) a rectangular luminous spot, withsides comprised between 4 mm and 9 mm.
 8. Method according to claim 1,characterised in that the laser beam (100) generates on the innersurface (12) a luminous spot which moves, relative to said surface (12),at a speed comprised between 20 mm/s and 40 mm/s.
 9. Device forhardening, by means of laser, at least one portion of the inner surface(12) of a hole (11) made in a mechanical piece (10) of cast iron ofpredominantly ferritic matrix, characterised in that it comprises: alaser source (2) adapted to generate a laser beam (100), and project itin axial direction inside the hole (11) of the piece (10); a deviatormeans (4) of the laser beam (100) which, associated with said lasersource (2), is adapted to be inserted in the hole (11) of the piece(10), and to deviate the beam (100), projecting it onto the innersurface (12); and a support device (5) of the cast iron piece (10); saidsupport device (5) and deviator means (4) being adapted to rotate withrespect to each other, and to slide in axial direction with respect toeach other, until said portion to be treated of the inner surface (12)is entirely subjected to the laser beam (100).
 10. Device according toclaim 9, characterised in that the support element (5) comprises arotating mandrel (52) adapted to hold the piece (10) so that the axis ofthe hole (11) coincides with the rotation axis of the mandrel (52)itself, and to set it in rotation at a predetermined speed.
 11. Deviceaccording to claim 9, characterised in that the support element (5)comprises a motorised carrier (50) adapted to slide with reciprocatingmotion along a rectilinear direction, so to engage the piece (10) toslide in the direction defined by the axis of the hole (11).
 12. Deviceaccording to claim 9, characterised in that the deviator means comprisesa faceted body (4), provided with a reflecting surface (40) which issloped with respect to the direction of the laser beam (100) leaving thelaser source (2).
 13. Device according to claim 12, characterised inthat the faceted body (4) is realised in copper.
 14. Device according toclaim 12, characterised in that the reflecting surface (40) is madespecular by means of polishing.
 15. Device according to claim 9,characterised in that it comprises means adapted to cool the deviatormeans (4).
 16. Device according to claim 9, characterised in that itcomprises means adapted to convey a flow of protective gas which,leaving a supply nozzle (6), is adapted to hit the deviator means (4)and the surface (12).